Off-highway vehicles of the type for which the present invention is intended are typically very large earth-moving machines that use an internal combustion engine, such as a diesel engine, to drive an alternator that produces electric power. The wheels of the vehicle are propelled by electric motors built into the wheels that are powered from the on-board alternator. See, for example, U.S. Pat. No. 3,897,843 for a general description of such a vehicle wheel. Because the vehicles are propelled by electric motors, the internal combustion engine is not available to assist in slowing the vehicle when the vehicle is switched into a coast mode. Accordingly, such vehicles typically include a dynamic braking circuit that can be switched into circuit with the electric motors. During this time, the momentum of the vehicle drives the motors and the motors are biased so as to act as electric power generators. The amount of retarding force that can be produced by the motors while acting as generators is a function of the amount of current produced by the motors.
When the electric motors of a vehicle are operating in a power generation mode to create a retarding effect on the vehicle, the power generated by the motors must be dissipated and this is usually accomplished by directing the power into a resistive grid, sometimes referred to as a dynamic braking grid. Since it is not desirable to dissipate power in the grid during normal propulsion of the vehicle, the grid is provided with switching devices that can switch the grid into and out of circuit with the motors depending upon the mode of operation of the vehicle, i.e., whether the vehicle is in a propulsion mode or a dynamic braking mode. In electric vehicle applications, the amount of power that may be generated, when measured in terms of voltage and current, can be significant. Motor currents may be in the range of several hundreds of amperes and motor voltage may exceed one thousand volts. Relatively large contactors are therefore used to switch the dynamic braking grid into and out of circuit with the motors.
The contactors used in the above described vehicle applications are generally fast acting solenoid actuated contactors such as a Siemens model 41A296327ALP2 contactor. This type contactor has been shown to clear a contact arc in several milliseconds. However, in off-highway vehicle operation, it is typical for the vehicle operator to cycle the vehicle between propulsion, coast and braking modes on a frequent basis. As a result, the contact tips on these contactors experience significant wear from repetitive arcing at the tips and must be replaced to prevent contactor failure. It is desirable to maximize the time between contactor replacement both to minimize cost of replacement as well as to minimize down-time of the vehicle.